1. Field of the Invention
The present invention relates to a receiver for receiving signals modulated by quadrature amplitude modulation (QAM) and, more particularly, to a QAM receiver and a carrier recovery method for receiving QAM signals and recovering the frequency offset and the phase jitter of carrier waves.
2. Description of the Related Art
The transmission methods of digital TV""s are largely classified into two methods: the one is the vestigial side band (VSB) method using a single carrier and the other is the coded orthogonal frequency division (OFDM) method using multiple carriers.
The OFDM method using multiple carriers readily recovers the signals damaged by multi-path channels and, unlike the VSB method using a single carrier, supports a single frequency network.
OFDM data are mapped by the QAM method prior to being transmitted, which transmission method is widely used for cable TV""s in U.S.A.
A QAM receiver receives radio frequency (RF) signals, mapped by the QAM method, via a tuner to perform a data recovery. Meanwhile, the tuner or the RF generator incurs frequency offset of several hundreds of KHz and phase jitter, which have to be minimized in order to achieve an accurate data recovery. The acquisition/tracking procedure for minimizing frequency offset and phase jitter is called xe2x80x9ccarrier recoveryxe2x80x9d.
FIG. 1 is a schematic block diagram showing the structure of a carrier recovery device in the conventional QAM receiver, in which the carrier recovery device includes a polar decision-oriented phase error detector 101, a loop filter 101d, and a numerical control oscillator (NCO) 101e. The polar decision-oriented phase error detector 101 includes a mixer 101a, a decider 101b, and a polar decision-oriented phase error generator 10c. 
Referring to FIG. 1, the mixer 101a of the polar decision-oriented phase error detector 101 demodulates a passband digital signal having frequency offset and phase jitter, generated by a preprocessing unit 100, with sine/cosine waves generated by the numerical control oscillator 101e to produce a baseband digital signal (RI, RQ) with the frequency offset and phase jitter recovered.
The decider 101b generates a deciding signal character (DI, DQ) conformable to the individual signal level of the baseband digital signal (RI, RQ) demodulated by the mixer 101a. 
For example, when the baseband digital signal (RI, RQ) falls in the deciding region of the first quadrant in the QAM character diagram of FIG. 2, the decider 101b generates a deciding signal character (DI, DQ) judging that the baseband digital signal (RI, RQ) is present in the first quadrant.
The polar decision-oriented phase error generator 101c detects a phase error by using the baseband digital signal (RI, RQ) demodulated by the mixer 101a and the deciding signal character (DI, DQ) generated from the decider 101b. 
Namely, the polar decision-oriented phase error generator 101c calculates the difference between the phase xcex8 of the demodulated baseband digital signal (RI, RQ) and the phase xcfx86 of the deciding signal character (DI, DQ), and detects the polarity of the phase difference. The characteristic function e(xcfx86) of the polar decision-oriented phase error generator 101c can be expressed by the following equation 1. Diagrams (a) and (b) of FIG. 2 represent the geometrical characteristics of e(xcfx86).
Diagram (a) of FIG. 2 shows that the result of e(xcfx86) has a positive value, i.e., sgn(xcex8xe2x88x92xcfx86) greater than 0 because the phase xcex8 of the demodulated signal character is greater than the phase xcfx86 of the deciding signal character. Contrarily, diagram (b) of FIG. 2 shows that the result of e(xcfx86) has a negative value, i.e., sgn(xcex8xe2x88x92xcfx86) less than 0 because the phase xcex8 of the demodulated signal character is less than the phase xcfx86 of the deciding signal character.
e(xcfx86)=sgn(xcex8xe2x88x92xcfx86)=sgn(RQ*DIxe2x88x92RI*DQ)xe2x80x83xe2x80x83[Equation 1]
In the equation 1, the sng(#) operator represents an operator for detecting the polarity of #; RI and RQ the in-phase and quadrature components of the demodulated signal character, respectively; xcex8 the phase of the demodulated signal character; DI and DQ the in-phase and quadrature components of the deciding signal character, respectively; xcfx86 the phase of the deciding signal character.
FIG. 3 is a detailed block diagram showing the hardware configuration of the above mechanism, i.e., polar decision-oriented phase error generator 101c. 
In the polar decision-oriented phase error generator 101c, multipliers 301 and 302 and subtracter 303 determine the phase error between the deciding signal character (DI, DQ) generated by the decider 101b and the demodulated signal character (RI, RQ) generated by the mixer 101a, according to the equation 1. Polarity detector 304 detects the polarity from the phase error determined by the substracter 303. Accordingly, the polar phase error e(xcfx86) thus detected has a value of +1, 0 or xe2x88x921. The output of the polar decision-oriented phase error generator 101c is fed into the loop filter 101d. 
The loop filter 101d, which uses a general primary baseband loop filter, cumulates the phase error e(xcfx86) detected by the polar decision-oriented phase error generator 101c to generate an intermediate frequency xcfx89c as the sum of the frequency offset xcex94xcfx89 and the phase jitter xcex94xcex8.
The numerical control oscillator 101e generates sine and cosine waves of which the center frequency is the intermediate frequency xcfx89c generated by the loop filter 101d. The sine and cosine waves are output to the mixer 101a. 
However, the conventional carrier recovery device applies the polar phase error e(xcfx86) to all characters regardless of the magnitude of the deciding signal character, i.e., DI2+DQ2, so that the phase jitter of the demodulated signal character increases with an increase in the magnitude of the deciding signal character DI2+DQ2, as shown in FIG. 9. That is, the demodulated signal character has a phase jitter that increases with an increased distance from the origin. This results in a deterioration of the signal-to-noise ratio (SNR) performance of the receiver, i.e., a deterioration of the acquisition/tracking performance even at a relatively small input SNR.
Such a narrow acquisition/tracking range not only causes a need of using a high-quality tuner of excellent mechanism, thus raising the expense of the tuner, but also leads to a deterioration of the BER performance of the receiver due to the great residual phase jitter.
It is, therefore, an object of the present invention to solve the problems with the prior art and to provide a QAM receiver and a carrier recovery method in which a weighted value variable depending on the magnitude of the deciding signal character is applied to the polar phase error so as to stably detect the accurate phase error.
To achieve the above object of the present invention, there is provided a QAM receiver including: a signal generator for multiplying the passband digital signal by a sine/cosine wave into a demodulated baseband digital signal, and generating a deciding signal character conformable to the individual signal level of the demodulated baseband digital signal; a first phase error measurer for calculating the phase difference between the demodulated baseband digital signal and the deciding signal character, and detecting the polarity of the phase difference to be output as a first phase error; a second phase error measurer for using the first phase error detected by the first phase error measurer to output a second phase error having a weighted value; and a filter and an oscillator for cumulating the received second phase error, and generating a sine/cosine wave proportionate to the cumulated phase error, the sine/cosine wave being output to the signal generator.
The second phase error measurer includes: a weight information generating decoder for calculating the magnitude of the received deciding signal character, and generating weight information proportionate to the magnitude of the deciding signal character; a first memory for storing positively weighted phase errors inversely proportionate to the weight information; a second memory for storing negatively weighted phase errors inversely proportionate to the weight information; a first selector using the weight information as a selection signal to selectively output one of the positively weighted phase errors stored in the first memory; a second selector using the weight information as a selection signal to selectively output one of the negatively weighted phase errors stored in the second memory; and a third selector using the first phase error as a selection signal to selectively generate the output of the first or second selector, or xe2x80x9c0xe2x80x9d as a second phase error.
The magnitude of the deciding signal character is the vector size from the origin to the deciding signal character (DI, DQ).
The weight information generating decoder decodes the deciding signal characters of the same radius into the same weight information.
The first and second memories store normalized positively and negatively weighted phase errors, respectively.
According to another aspect of the present invention, there is provided a carrier recovery method for the QAM receiver including the steps of: (a) multiplying the passband digital signal by a sine/cosine wave into a demodulated baseband digital signal, and generating a deciding signal character conformable to the individual signal level of the demodulated baseband digital signal; (b) calculating the phase difference between the demodulated baseband digital signal and the deciding signal character, and detecting the polarity of the phase difference to be output as a first phase error; (c) using the first phase error as a control signal to output a second phase error having a weighted value; and (d) cumulating the received second phase error, and generating a sine/cosine wave proportionate to the cumulated phase error, the sine/cosine wave being output to the step (a).
The step (c) includes the steps of: determining the magnitude of the received deciding signal character, and generating weight information proportionate to the magnitude of the deciding signal character; storing positively weighted phase errors inversely proportionate to the weight information; storing negatively weighted phase errors inversely proportionate to the weight information; using the weight information as a selection signal to selectively output one of the positively weighted phase errors stored in the first storing step; using the weight information as a selection signal to selectively output one of the negatively weighted phase errors stored in the second storing step; and using the first phase error as a selection signal to selectively generate the output of the first or second selecting steps, or xe2x80x9c0xe2x80x9d as a second phase error.
The present invention applies a weight value variable depending on the magnitude of the deciding signal character to the polar phase error so that the phase jitter has a constant shape regardless of the magnitude of the deciding signal character.